255 Insights on the fate of protein-bound and free amino acids and importance to protein metabolism in sows

Abstract Amino acid requirements for sows are dynamic throughout the reproductive cycle and, relative to early- and mid-gestation, increase in late gestation due to exponential protein deposition in mammary and fetal pools and during lactation due to milk production. In order to meet estimated requirements via the diet, a sufficient quantity of amino acids must be supplied in the correct balance, at the right time. Protein accretion, however, occurs continuously in pregnancy-associated and milk protein pools for pregnant and lactating sows, respectively, which is incongruent with the feeding frequency of sows. Sows are provided restricted amounts of a low-protein diet once or twice per day in gestation whereas, lactating sows, if offered feed ad libitum, receive higher-protein diets but will still consume ‘meals’ mainly throughout daylight hours. Amino acid supply is further complicated by the large maternal protein reservoir that can be liberated in both gestation and lactation to buffer dietary deficiencies in terms of amino acid quantity and balance, but also in timing of amino acid release from dietary sources during digestion and absorption. We found that plasma concentrations of amino acids following a polynomial relationship in the six hours following consumption of a meal, seemingly representing amino acids entering blood from digested protein and subsequently, maternal protein mobilization by the lactating sow. Thus, the digestibility characteristics of dietary protein can influence amino acid utilization by the reproductive sow. Indeed, using a greater proportion of slowly degradable dietary protein reduced body protein loss by lactating sows, presumably due to a more sustained supply of available amino acids for milk protein synthesis, which reduced reliance on maternal protein mobilization. Moreover, plasma flow to the mammary gland and amino acid uptake by the mammary gland increased when lactating sows were in a fed state, demonstrating some fluidity in mammary gland metabolism in response to available nutrients, and yet remarkably consistent casein output, regardless of the dietary amino acid balance and time relative to a meal. Better understanding the dynamics of amino acid digestion, absorption, and utilization by reproductive sows will help researchers to synthesize and assess feeding approaches that maximize nutrient utilization efficiency and sow productivity.

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